A026646 a(n) = Sum_{0<=i,j<=n} A026637(i,j).

1, 3, 7, 17, 37, 79, 163, 333, 673, 1355, 2719, 5449, 10909, 21831, 43675, 87365, 174745, 349507, 699031, 1398081, 2796181, 5592383, 11184787, 22369597, 44739217, 89478459, 178956943, 357913913, 715827853, 1431655735

Offset

0,2

Comments

a(n) indexes the corner blocks on the Jacobsthal spiral built from blocks of unit area (using J(1) and J(2) as the sides of the first block). - Paul Barry, Mar 06 2008

Partial sums of A026644, which are the row sums of A026637. - Paul Barry, Mar 06 2008

Links

Table of n, a(n) for n=0..29.

Formula

G.f.: [1-x^2+2x^3]/[(1-x)(1-2x)(1-x^2)]. - Ralf Stephan, Apr 30 2004

a(n)=J(n+3)-2*floor((n+2)/2), where J(n)=A001045(n) the Jacobsthal numbers; a(n)=1+sum{k=0..n, J(k+2)-1}; - Paul Barry, Mar 06 2008

a(n+1) = 2*a(n) + A109613(n), a(0)=1. - Paul Curtz, Sep 22 2019

Mathematica

CoefficientList[Series[(1-x^2+2x^3)/((1-x)(1-2x)(1-x^2)), {x, 0, 29}], x] (* Metin Sariyar, Sep 22 2019 *)

Crossrefs

Cf. A000126.

Cf. A026637, A026644, A109613.

Sequence in context: A244629 A065545 A110930 * A026668 A111210 A033489

Adjacent sequences: A026643 A026644 A026645 * A026647 A026648 A026649

Keyword

nonn

Author

Clark Kimberling

Status

approved